Two-dimensional phase Doppler anemometry measurements have been carried out to determine the size and velocity distributions of electrosprayed droplets generated from methanol-water mixtures. We investigated spraying conditions close to those of electrospray ionization sources. The droplet size and the axial and radial velocity distributions were measured as a function of liquid flow rate, needle-to-counter electrode distance, bias voltage, position of the probe volume, and electrical conductivity of the liquid. In 90:10 (v/v) methanolwater mixtures the droplet size decreased from ∼7 to ∼1.6 µm as the conductivity increased as a consequence of a 3 orders of magnitude increase in ionic strength. As the position of the probe volume was moved along the spray axis, two different spray dynamics were observed. Solutions of low conductivity (c < 10 -5 M) on the average produced 5.5 µm droplets at the capillary that gradually decreased to 4.0 µm as the drops moved away from the tip. Solutions of higher conductivity (c > 10 -3 M), however, resulted in smaller droplets at the needle (1.6 µm) that increased in size to 4.2 µm as the particles traveled toward the counter electrode. The droplet size reduction can be explained by evaporation and/or Coulomb explosion, whereas the increase in droplet size may be the consequence of droplet segregation or coalescence. Axial velocity distributions show compression along the spray axis (e.g., observed at 10 mm ∼50% reduction of the width at 4 mm is seen). Moving downstream, the average velocity of droplets from water-methanol mixtures decreases monotonically. Solutions containing KCl exhibit a maximum in axial droplet velocity as the probe volume moves away from the capillary. These profile changes can be explained by differences in the electric field distribution along the spray centerline.

In this paper we present a model to explain matrixassisted laser desorption (MALD) of intact large molecules. According to preliminary estimates of energy deposition it seems likely that laser heating of the matrix up to the phase transition temperature is one of the prerequisites to carry out successful MALD experiments. The generated plume is described in terms of hydrodynamics. The results reflect several important features of the measurements: the velocity of neutral particles and the power law of total desorbed flux versus laser irradiance. Furthermore, expansion cooling of the plume is reported, rationalizing the absence of thermal degradation processes.

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Characterization of the metabolic heterogeneity in cell populations requires the analysis of single cells. Most current methods in single-cell analysis rely on cell manipulation, potentially altering the abundance of metabolites in individual cells. A small sample volume and the chemical diversity of metabolites are additional challenges in single-cell metabolomics. Here, we describe the combination of fiber-based laser ablation electrospray ionization (f-LAESI) with 21 T Fourier transform ion cyclotron resonance mass spectrometry (21TFTICR-MS) for in situ single-cell metabolic profiling in plant tissue. Single plant cells infected by bacteria were selected and sampled directly from the tissue without cell manipulation through mid-infrared ablation with a fine optical fiber tip for ionization by f-LAESI. Ultrahigh performance 21T-FTICR-MS enabled the simultaneous capture of isotopic fine structures (IFSs) for 47 known and 11 unknown compounds, thus elucidating their elemental compositions from single cells and providing information on metabolic heterogeneity in the cell population.